Phosphonous acids are employing in preparing phosphonic acids and phosphinic acids and their derivatives thereof which are useful in preparing angiotensin converting enzyme inhibitors, as disclosed, for example, in U.S. Pat. No. 4,452,790 to Karanewsky, et al.
Until now, phosphonous acids have been prepared by high pressure reaction techniques wherein a high pressure bomb or autoclaves are employed as described by E. E. Nifantev and M. P. Koroteev, J. Gen. Chem. (U.S.S.R.), 37 1293 (1967); U.S. Pat. No. 2,726,256 to R. C. Morris and J. L. Van Winkle (assigned to Shell Development Co.), [C. A. 10124b, 50 (1956)]; N. V. de Bataafsche, Petroleum Maatschappig; British Pat. No. 660,918, Nov. 14, 1951, [C. A. 8145C, 46 (1952)]; U.S. Pat. No. 2,648,695 to C. W. Smith (assigned to Shell Development Co.), [C. A. 8252d, 48 (1954)].
In such prior art procedures, reactions were conducted in autoclaves (bombs) at high temperatures (120.degree.-150.degree. C.) and the products of the reaction presented a hard to separate mixture of alkylphosphonous and dialkylphosphinic acids with a total yield of 30-70%. The process of the present invention employs relatively mild conditions, namely, only refluxing in alcohol solvents in open vessel reactors to produce phosphonous acids in good yield and excellent purity.
U.S. Pat. No. 4,590,014 to Wolf et al discloses a method for preparing alkyl phosphinate salts by simultaneously adding an olefinic starting material (R--CH.dbd.CH.sub.2) and a free radical initiator, such as benzoyl peroxide or azobisisobutyronitrile, to an aqueous alcoholic solution of sodium hypophosphite and allowing the reaction to continue for 1.5 to 6 hours. Due to the presence of the sodium salt starting material, the reaction medium must be maintained under neutral conditions to avoid converting the sodium salt to the hypophosphorous acid.
U.S. Pat. No. 2,957,931 to Hamilton et al discloses a process for preparing organic phosphorus compounds wherein at least one organic radical is joined to a phosphorus by a direct carbon-phosphorus linkage which process involves the reaction of phosphorus compounds having at least one phosphorus-hydrogen linkage per molecule (including hypophosphorous acid and sodium hypophosphite) with organic compounds having at least one unsaturated carbon-carbon linkage per molecule (which include 1-olefins), which reaction is carried out at temperatures of from 20.degree. C. to 300.degree. C. in the presence of water or alcohol solvent. As indicated in column 26, where water is used as a solvent, alkaline conditions would be preferred, where an alcohol solvent is employed, the alkyl ester is obtained.
U.S. Pat. No. 4,108,889 to Connor discloses a method for preparing alkane phosphonic acids by reacting an .alpha.-olefin with a macroreticular strong acid cation exchange resin to isomerize the olefin to form an internally unsaturated olefin, reacting such olefin with dimethyl phosphite in the presence of free radical initiator, such as di-t-butylperoxide, di-benzoylperoxide or azobisisobutyronitrile to produce non-terminally substituted C.sub.12 -C.sub.22 alkanephosphonic acid dimethyl ester and demethylating the dimethyl ester with HCl or HBr.
U.S. Pat. No. 4,374,780 to Robertson discloses a method for preparing di-2,4,4'-trimethylpentylphosphinic acid by free radical addition of two moles of 2,4,4'-trimethylpentene-1 to phosphine employing high phosphine pressures of up to 100 psig, followed by oxidation with two moles of hydrogen peroxide. Azobisisobutyrylnitrile is employed as a free radical initiator.